1. Field of the Invention
The present invention relates in general to an electrochemical device, and more particularly to an electrochemical device which has an integrally formed heating portion for heating an electrochemical cell.
2. Discussion of the Prior Art
A sensor which employs an oxygen-ion conductive solid electrolyte material such as zirconia ceramic is known as an oxygen sensor operable according to an oxygen concentration cell, for determining the concentration of oxygen contained in emissions produced by internal combustion engines for motor vehicles, or industrial furnaces or boilers. Other electrochemical devices similar to such oxygen sensors, which utilize the principle of a concentration cell, are also known, as detectors for detecting hydrogen, nitrogen or carbon dioxide, or as pumps associated with these components.
The electrochemical device as indicated above includes an electrochemical cell which consists of a suitable solid electrolyte and at least one pair of electrodes formed in contact with the solid electrolyte, as well known in the art. For assuring an accurate and reliable operation of the cell of the electrochemical device even while the temperature of a gas to be measured is relatively low, various heating means or heat-generating members have been proposed to heat a detecting portion of the electrochemical cell, i.e., a portion at which the electrodes are disposed, so that the detecting portion is maintained at a suitable operating temperature. Such heating means or heat-generating members are provided as an integral part of an electrochemical element which has the electrochemical cell.
However, a current applied to such integrally built-in heating means to heat the electrochemical cell may leak toward the cell, and the output of the cell may be adversely influenced by the leak current. While the use of a suitable electrically insulating layer between the electrochemical cell and the heating means is proposed in the art, the insulating layer alone cannot be a satisfactory solution to the above problem.
In light of the above problem, an arrangement employing a protective or shielding electrode between the electrochemical cell and the heating means is disclosed in U.S. Pat. No. 4,400,260 (DE No. P3120159.8) and Japanese Patent Application Laid-open Publication No. 59-197851 (published in 1984), for example. In the disclosed arrangement, the protective electrode is embedded in a portion of a solid electrolyte body of the electrochemical cell or formed in contact with the solid electrolyte body, and the heater or heating means is disposed on the solid electrolyte body via an electrically insulating layer. Alternatively, electrically insulating layers are provided on opposite sides of a measuring electrode which is adapted to also function as the protective electrode. In this case, the heater is disposed on one of the two insulating layers, while the solid electrolyte body of the electrochemical cell is diposed on the other insulating layer. In either case, the electrochemical element is adapted so that the leak current from the heater flows to the protective electrode.
However, the above-described known arrangement using the protective electrode has various potential problems that should be solved. For instance, where the protective electrode is disposed in contact with the solid electrolyte body of the electrochemical cell, there may be induced an electromotive force between the protective electrode and one of the electrodes of the electrochemical cell, based on a difference in the oxygen partial pressure between the atmospheres contacting these two electrodes, if the two electrodes are electrically connected to each other.
In the case where the protective electrode is embedded in the solid electrolyte body or covered by a layer having a high gas-diffusion resistance, ions flow from a portion of the solid electrolyte body surrounding the protective electrode, due to the leak current from the heater, whereby that solid electrolyte portion is deteriorated.
In the case where the electrochemical element includes two electrically insulating layers which are disposed on the opposite sides of the protective electrode and are interposed between the solid electrolyte body of the electrochemical cell and the heater, the surface area of the protective electrode should be large enough to shield the leak current from the heater. That is, if the heater has a size sufficiently large for heating the electrochemical element to a desired operating temperature, the protective electrode should be accordingly large-sized. In this case, the cost of manufacture of the electrochemical element whose laminar structure includes the electrochemical cell, heater, protective electrode and insulating layers is inevitably increased. Further, the strength and durability of the electrochemical element, which is substantially an integral ceramic mass, are reduced due to the presence of the protective electrode in the ceramic mass, which electrode is substantially a metal layer heterogeneous to the ceramic mass of the electrochemical element. A solution to these problems requires the use of a heater which has a relatively small surface area for heat generation, i.e., a relatively small heating capacity.